1. Technical Field
The inventions relate to electro-optical modulators and methods of operating electro-optical modulators.
2. Discussion of the Related Art
This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Some optical modulators are based on a Mach-Zehnder interferometer (MZI) that is configured for push-pull operation. For push-pull operation, both of the internal optical branches of the MZI has an optical phase shifter thereon, and the optical phase shifters are operated together to modulate data onto an optical carrier in the MZI. The optical phase shifters can be operated together to cause relative phase shifts between light output by the two internal optical branches such that said light interferes in an output optical coupler to form a 0th order or fundamental optical propagation mode for which the output waveguide attached thereto guides the light to an output of the MZI. In such a configuration, the interfered light may propagate to an optical circuit attached to the output of the MZI, and the MZI is in an ‘ON’ state. Alternately, the optical phase shifters can be operated together to cause the relative phase shifts between light output by the two internal optical branches such that said light interferes in the output optical coupler to form a 1st order optical propagation mode for which the output waveguide will not guide the interfered light to the output of the MZI. In such a configuration, the interfered light typically does not substantially propagate to an optical circuit attached to the output of the MZI, and the MZI is in an ‘OFF’ state. In the ‘ON’ state, light from the internal optical branches can be referred to as constructively interfering within the MZI, and in the ‘OFF’ state, light from the internal optical branches may be referred to as destructively interfering in the MZI.
In some push-pull operated MZIs, the same central electrode drives both optical phase shifters of the two internal optical branches of the MZI. In particular, the central electrode can cause optical phase shifts of opposite sign in the two internal optical branches. To simultaneously generate phase shifts of opposite sign, the central electrode may forward bias a semiconductor junction in one internal optical branch of the MZI and simultaneously reverse bias a semiconductor junction in the other internal optical branch of the MZI. For example, a voltage applied to the central electrode can produce a phase shift of about π/2 in one internal optical branch and simultaneously produce a phase shift of about −π/2 in the other internal optical branch.
Such push-pull methods of operation can provide advantages over non-push-pull methods of operation. For example, push-pull operation can lower the average energy consumption by an MZI during operation as an optical amplitude modulator. That is, push-pull operation may be performed with lower control voltages than non-push-pull operation so that time-averaged operating powers are lower when an MZI-based optical modulator is push-pull operated than when a MZI-based optical modulator is operated by modulating a voltage to a single optical phase shifter.